4 doctors weighed in:

Hi, I just want to ask about the ultrasound for pregnancy. Is it possible to determine in ultrasound how old the baby is inside?

4 doctors weighed in
Dr. Nikolaos Zacharias
Obstetrics & Gynecology - Maternal Fetal Medicine
1 doctor agrees

In brief: With great precision

An early ultrasound is the most precise (+/- 3-5 days until 8 weeks) in the determination of the due date and therefore the fetal age.
This declines to +/-5-7 days @8-12 weeks +/-7-10 days @13-23 weeks +/-10-14 days @24-28 weeks +/-14-17 days @29-32 weeks and +/- 17-21 days thereafter. Due to the biological variation of the ovulation date ultrasound <24 weeks is more reliable than last period!

In brief: With great precision

An early ultrasound is the most precise (+/- 3-5 days until 8 weeks) in the determination of the due date and therefore the fetal age.
This declines to +/-5-7 days @8-12 weeks +/-7-10 days @13-23 weeks +/-10-14 days @24-28 weeks +/-14-17 days @29-32 weeks and +/- 17-21 days thereafter. Due to the biological variation of the ovulation date ultrasound <24 weeks is more reliable than last period!
Dr. Nikolaos Zacharias
Dr. Nikolaos Zacharias
Thank
Dr. Danae Steele
Obstetrics & Gynecology - Maternal Fetal Medicine
1 doctor agrees

In brief: Yes

Ultrasound is one of the best ways to determine how far along a pregnancy is , but it is much more accurate in the early part of a pregnancy than later. An ultrasound done in the first trimester (up to 12 weeks of pregnancy) is accurate to within 7 days.
In the 3rd trimester (the last 3 months of pregnancy) it is only accurate to within 3 weeks for determining how far along the pregnancy is.

In brief: Yes

Ultrasound is one of the best ways to determine how far along a pregnancy is , but it is much more accurate in the early part of a pregnancy than later. An ultrasound done in the first trimester (up to 12 weeks of pregnancy) is accurate to within 7 days.
In the 3rd trimester (the last 3 months of pregnancy) it is only accurate to within 3 weeks for determining how far along the pregnancy is.
Dr. Danae Steele
Dr. Danae Steele
Thank
Dr. Marilynn Frederiksen
Obstetrics & Gynecology - Maternal Fetal Medicine

In brief: Gestational age

The ultrasound will take measurements of the fetus.
Those measurements are used to determine fetal age. The best estimate of gestational age is in the first trimester when the error is within + or - 3 days. In the early second trimester the estimate has an error of plus or minus one week. Later in pregnancy the error increases to + or - two or three weeks. So first trimester is best for dating.

In brief: Gestational age

The ultrasound will take measurements of the fetus.
Those measurements are used to determine fetal age. The best estimate of gestational age is in the first trimester when the error is within + or - 3 days. In the early second trimester the estimate has an error of plus or minus one week. Later in pregnancy the error increases to + or - two or three weeks. So first trimester is best for dating.
Dr. Marilynn Frederiksen
Dr. Marilynn Frederiksen
Thank
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